(1) Field of Invention
The disclosure relates to a system and method in the field of mobile communication.
(2) Description of Related Art
The disclosure relates to a method of setting up a wireless long range communication network between several units communication units. Also disclosed is a system for setting up a wireless long range communication network with integrated positioning.
The dominating wireless system deployments today are telecom systems optimized for RF bandwidth efficiency and low manufacturing costs for the user terminals. Examples of such systems are conventional digital GSM telecommunication systems and 3G, HSDPA, WiMAX, LTE and other 4G systems for mass distribution of digital bandwidth.
Common for all these systems are that the communication system structures are cell based with a base station acting as a master that controls the traffic in each cell or sector. In the context of RF bandwidth efficiency it is not optimum to combine short rage traffic with long range traffic because the communication parameters are optimized for a given range of time-of-flight, and hence, different cell sizes are combined from pico-cells to macro-cells. For GSM the maximum macro-cell radius is 35 km. In order to re-use the RF frequency resource and increase capacity for mass distribution, smaller cell radius are used, a typical cell radius is 1-5 km.
For 4G systems such as LTE even smaller cells radiuses are defined. To increase bandwidth efficiency multiple antenna elements and MIMO techniques are used, but these techniques increase efficiency only when propagation paths through different reflections can be utilized, and when the channel is not changing rapidly, and hence for short range in reflective environments the MIMO techniques are efficient. The number of MIMO antennas on the base station is limited by processing power and cost-benefit compromises, and the number of antenna elements is typically in the range between 3 and 8. For the client terminal, the available space for antenna elements, available processing power, current consumption and manufacturing cost is limited, and the number of antenna elements is typically in a typical range of 2-3 elements.
For mobile applications at longer ranges, the channel changes rapidly causing large channel dispersion, and hence the efficiency of both MIMO techniques and bandwidth efficiency is severely reduced when the terminal is moving. In rural areas without coverage from cell based systems, satellite systems are used. Examples of satellite systems are VSAT and IRIDIUM. Common for all satellite systems are that all traffic has to be routed through the satellite, and hence, the system is a wireless star topology system where the frequency resources are shared by all user terminals covered by the satellite. The long distance to the satellite leads to a need for highly directive antennas, such as motorized parabolic dishes with gyro stabilization or other compromises like bandwidth reduction, to achieve a stable link for mobile installations.
The satellite systems also suffer from additional atmospheric losses and dispersion effects for terminals close to the poles of the earth, due to the inclination angle is low. The impact of the atmospheric losses and dispersion effects is known to result in partial unstable links.
Another main limitation with satellite systems are the long delays because of the long distance to the satellite. A typical accumulated delay to and from a satellite in geostationary orbit is 240 ms. For applications, such as voice and video conferencing and real-time remote control applications, this delay reduces the quality of service, link efficiency and also excludes real-time applications that requires short latency time in order to operate properly.
Real time regulation systems related to navigation, such as guidance systems for airplanes or missiles, require fast regulation loops. These are implemented as a combination of separate sensor systems and communication systems. Sensor systems for guidance involving GPS have a limited update speed and a limited vertical resolution in some satellite constellations that limits the performance for high speed guiding systems. Satellite positioning services, such as GPS, could also easily be disrupted by jamming transmitters which is a severe limitation of secure operation. Optical systems are fast, precise and difficult to disrupt by jamming, but cannot operate at long distances in unfavorable weather conditions, such as snow, rain and fog.
With prior art combination of different sensor systems and communication systems that must be applied to implement a real-time guidance system for mobile units, this will introduce extra latency and complexity increasing regulation loop delay and reduced stability margins.
GB2448510A discloses a radio frequency communication method, apparatus or system which comprises a first antenna which transmits information regarding its location to a second antenna which receives the said information and uses it to align a directional radiation beam from the second antenna towards the location of the first antenna. The publication describes a lobe-aligning system based on switching antennas between omnidirectional antennas transmitting location information and highly directive antennas which are used for communication purposes. The system requires a secondary radio system which is complex and expensive in addition to a very precise direction sensor system in order to determine the optimum direction for the phased array antenna system. The beam will always be directed towards each other even if direct line-of-sight is blocked and communication could be possible via a reflection of the signal. The speed of the mobile units and number of units in the network are limited by the network capacity of the first antenna system which is transmitting omnidirectional at a relatively low data rate.
WO2010025996 discloses a method for performing communications in a wireless communication network, comprising receiving mobility information about at least one moving mobile station in a mobility server of the wireless communication network, using the mobility information from the mobility server for calculating antenna weights to be applied to antenna elements of an antenna array for steering a beam generated by the antenna array to the moving mobile station. The publication describes an antenna array ground system and a RF transceiver system on the mobile units. The mobility data from the mobile units are communicated through a network to a mobility server which sends the information to the steering vector for the antenna array. The system is complex and if the system has many nodes in the network, the latency of mobility information distribution will be significant. If the RF channel is rapidly changing and the optimum antenna direction is not the direct line-of-sight to the mobile station, the described system will have significant limitations with respect to reliability and range. The publication does not describe a symmetrical array antenna solution for both the ground station and the aircraft, and limits the frequency re-use for the elevated mobile stations that does not incorporate narrow, adaptive antenna beams.
US20030174048 discloses an identification tag with RF circuitry, ultra-wide bandwidth (UWB) circuitry and a method is described for measuring the time-of-flight between the identification tag and a local device. By combining the measured time-of-flight distances between several units and applying triangulation, the relative position of the identification tag can be found. The disclosed publication uses signal processing in the time domain only to determine the time-of-arrival, and hence, the time-of-flight measurement are very sensitive to inaccuracies by delay spread from a RF multipath scenario. If the delay spread is short, the numerous multipath copies of the signal cannot be separated using the disclosed correlation method. The utilization of the disclosed UWB circuitry has other major disadvantages, such as large current consumption, high-speed analog to digital converters with limited dynamic range and because the RF bandwidth is very large, the allowed transmission output power and operative range is very limited.
EP0837567 discloses an airborne broadband communication network. A deployed group of airborne vehicles provide relay communication service among mobile and airborne customers by using a directed phased array antenna. Each airborne vehicle has a wireless link to one or more neighboring airborne vehicles forming an airborne inter-networked mesh constellation for routing traffic between mobile customers. The phased array antenna is implemented by phase delay elements on each antenna element, and hence, the system must have a pre-defined anticipation of the direction of the in-coming signal by other means, such as sensors, for attitude information in order to direct the receiver beam in the right direction before data can be demodulated with maximum gain. For non-line of sight applications this method is not feasible because the direction vary very rapidly. For line of sight applications, the method limits the useable bandwidth if a beam lock is lost, and the receiver needs to re-lock the antenna beam in the receiver.